I have tendinitis in my right wrist and there are some days when it hurts to knit and I have to put it down. I didn't get the tendinitis from knitting; I got it as a repetitive motion injury at an old job I used to have, and it flares up from time to time. Add to that, at the same time I got the tendinitis, I also sprained it, too. I have to stretch the muscle sometimes to loosen it up. When the weather is humid, it's about to storm (rain or snow) or at certain times in the spring or fall, I can't knit at all.
I have to put it away until the weather settles down.